Evangelical Christians have just delivered Donald Trump — the Republican presidential candidate most out of sync with their biblical values — a resounding victory in South Carolina. Of the 65 percent of Republican voters who identify as evangelicals, a third of them cast their ballot for Trump, more than any other candidate. Why?

For roughly the same reason that a medieval pope, Leo III, anointed another political bully, Charles the Great, better known as Charlemagne. Put simply, they want a “protector in chief.” Facing a political culture increasingly hostile to their beliefs — and a government riding roughshod over their religious freedoms — evangelicals believe Mr. Trump will be the best guardian of their liberties.

“Trump is a fighter,” Mark Burns, pastor of the Greenville, S.C.-based Christian Television Network, told Fox News. “He is the one to fight for Christianity and for our conservative values we hold dear.”

That’s what Pope Leo believed about Charles, king of the Franks, when he personally crowned him king of the Holy Roman Empire on Christmas Day, A.D. 800. Leo had become so unpopular in Rome that in 799 a band of assassins attacked him during a sacred procession and tried to cut out his eyes and tongue. Leo managed to escape and fled immediately to Charles, known as a defender of the church, and asked him to drive his enemies out of the city.

The king was happy to comply: On Dec. 24, 800, he sent the pope back to Rome with an armed bodyguard, with himself marching “in full martial array” into the city. As Charles promised: “My task, assisted by divine piety, is everywhere to defend the Church of Christ.” ...